Study On The Drying Technology And Storage Stability Of Yacon Powder

Effect of five different drying methods on the quality characteristics of yacon powder were investigated to obtain the appropriate drying technology of yacon powder. Moisture adsorption characteristics and glass transition temperature of yacon powder (with and without the addition of maltodextrin) are researched. The effect of two preservation theory, namely water activity (aw) preservation theory and glass transition theory on the storage stability of yacon powder was discussed. The results will provide theoretical basis and technical reference for yacon powder processing, storage and preservation. The main research contents and conclusions are as follows:1 In order to obtain a better drying method for yacon powder, this paper compares the effects of hot air drying (HA), vacuum drying (VD), freeze-drying (FD), heat pump and microwave combination drying (HPD+MW), hot air and microwave combination drying (HA+MW) on the quality of yacon powder by means of the determination of color, bulk density, angle of repose, hygroscopicity degree of caking, dissolution time and water holding capacity. The results show that yacon powder dried by FD had the best color with the lowest bulk density, angle of repose, hygroscopicity, degree of caking, the shortest dissolution time and the lowest water holding capacity.The quality of yacon powder obtained by VD ranked only second to FD but higher than other drying methods followed by HPD+MW and HA+MW. The worst quality of yacon powder was acquired by HA. On the whole, VD is the optimum drying method to obtain yacon powder with overall consideration of yacon powder qualities, energy consumption of drying system and drying efficiency.2. Moisture adsorption isotherms of yacon powder (YP) and yacon powder with maltodextrin (YP-MD) (solids ratio of yacon powder:maltodextrin=1:1) were determined at different temperatures (15,25 and 35℃) using a static gravimetric method. Seven kinds of commonly used mathematical model is used for the fitting of the experimental data to obtain the optimum model for the description of the adsorption characteristics of YP. Then, discussing the thermodynamic properties (net isosteric heat of adsorption, adsorption differential entropy and entropy-enthalpy theory, etc.) of YP and effects of maltodextrin on adsorption characteristics and thermodynamic properties of YP. The results show that the adsorption isotherms of YP and YP-MD shows a "J" type; in a specific temperature range, with the increase of water activity, equilibrium moisture content (EMC) of YP and YP-MD increase. The monolayer moisture of YP and YP-MD decreases from 15.04g/100g(15℃) to 9.04g/100g(35℃), from 8.80g/100g(15℃) to 5.91g/100g(35℃), respectively, so MD reduces the monolayer moisture of YP. Within the range of the temperature (15 to 35℃) and aw (0.11 to 0.92), GAB model is the fittest model for description of moisture adsorption characteristics of YP and YP-MD; the net isosteric heat of adsorption decreases with the increase of moisture content; the net isosteric adsorption heat of YP and YP-MD decreased from 67.44 to 62.232 kJ/mol and 66.167 to 62.121 kJ/mol, respectively, when the moisture content increased from 6.33 to 75.29g/100g dry matter; the differential entropy decreases with the increase of moisture content, and the value of differential entropy of YP is always higher than the value of YP-MD; the moisture adsorption processes of YP and YP-MD can be characterized as entropy driven; spreading pressure of YP and YP-MD increases with the increase of water activity, and decreases as the temperature increases; the value of spreading pressure of YP is always higher than the value of YP-MD, when the water activity and temperature of YP is the same as YP-MD.3 Differential scanning calorimetry (DSC) is used to determine the glass transition temperature (Tg) of yacon powder, and Gordon-Taylor equation is applied to the fitting of experimental data. Researching the effect of maltodextrin on Tg by adding maltodextrin, and establishing the relationship among the Tg-aw-Xw of YP and YP-MD. Then we can draw the corresponding critical water activity (CWA) and critical moisture content (CWC) of YP and YP-MD. The results show that the addition of MD makes CWC and CWA of yacon powder increase from 4.55g water/100g and 0.12 to 7.23g water/100g and 0.27 at 25℃. Tg of YP and YP-MD decreases from 15.78 to -72.56℃ and from 30.54 to -53.47℃, respectively, when water content of YP and YP-MD increases from 0.0595g/g(w.b.) to 0.3088g/g(w.b.) and from 0.0511g/g(w.b.) to 0.2766g/g(w.b.), respectively. The addition of MD makes the Tg increase from 15.78℃ to 30.54℃, when aw is 0.11. Therefore, the addition of MD can significantly improve the storage stability of the yacon powder.